The effect of a single dose of hydrocortisone on three different immunoregulatory functions influencing the magnitude of an antibody response of mice to Type III pneumococcal polysaccharide (SSS-III) was evaluated in the present work; these functions included suppressor, amplifier, and helper activity which are dependent upon the presence of distinct subpopulations of thymus-derived (T) lymphocytes. The results obtained show that a single injection of a relatively large dose of hydrocortisone eliminated both the generation and expression of helper T cell activity. But, under the same experimental conditions, suppressor T cell and amplifier T cell activities were uneffected, even in this steroid-sensitive species; hydrocortisone did not influence the capacity of bone marrow-derived precurosors of antibody-forming cells (B cells) to respond to SSS-III. Such selective sensitivity to steroid may account for some of the immunosuppressive potency of steroids.